Typically, variable pitch propeller systems include; a plurality of propeller blades extending radially from a hub, a mechanical pitch actuating system attaching to the blades, a control system for controlling the actuating system, and an engine for rotating the hub. In most modern aircraft, the actuating system is of the hydraulic variety. An example of such a system is shown in U.S. Pat. No. 4,523,891 to Schwartz et al and owned by the Assignee herein, such patent being hereby incorporated by reference.
As those skilled in the art will appreciate, a malfunction of the control system could result in engine overspeed conditions which may damage the components of the propeller system. The aircraft may have a backup control system to assume blade pitch control should such a malfunction occur.
Some pitch control systems have a main hydromechanical controller and a back-up hydromechanical controller. An example of a pitch control system having a back-up hydromechanical controller is provided in U.S. Pat. No. 4,588,354 commonly owned the Assignee herein and hereby incorporated by reference. Other pitch control systems may have a main hydromechanical controller and an electronic controller which acts as a back-up for the hydromechanical controller and as a main controller for ground operations (reverse pitch and the like). The electronic controller instructs the mechanical pitch actuation system to set the pitch of propeller blades as may be required. In such a system, if the electronic control fails, the propeller blades automatically go to feather.